1. Field of the Invention
The present invention relates to an information processing apparatus and GPS positioning method for measuring a position by utilizing the Doppler effect of a GPS (global positioning system) satellite.
2. Description of the Related Art
GPS positioning techniques utilizing a satellite (i.e., GPS satellite) are currently taken advantage of by diverse categories of information processing apparatuses such as a vehicle mounted apparatus, a mobile information terminal (e.g., personal digital assistant: PDA), a mobile phone, a PHS (Personal Handyphone System) and a PC (personal computer).
GPS positioning can largely be categorized into standalone positioning and interference positioning. The former is a method for acquiring no less than four GPS satellites, measuring pseudo-distances from a measuring point to the respective satellites, solving simultaneous equations containing four unknowns, and thereby calculating the position of the measuring point. Whereas the latter is a method for positioning by using a plurality of measuring points utilizing wave interference. The present invention relates to the former method, i.e., the stand alone positioning.
Navigation data of each GPS satellite used for GPS positioning is mainly categorized into almanac data and ephemeris data. The almanac data describes parameters for figuring out approximate positions of all the GPS satellites and can be used for about two weeks. The time limit is governed by the orbits of the respective GPS satellites changing with time and corresponds to the expiration date of the data.
The ephemeris data describes detailed parameters of satellite orbital information about each satellite and is used by an information processing apparatus for calculating the position of each satellite The time limit of the ephemeris data is about two hours.
FIG. 1 is an operation flow chart of a conventional stand alone positioning. First, an information processing apparatus receives a radio wave in a radio frequency (RF) band from a GPS satellite and converts it into a signal in an intermediate frequency (IF) band by down conversion (step 101).
Then it searches for a receiving frequency of the radio wave from the GPS satellite (step 102), in which information from a target satellite is extracted by multiplying a signal of the receiving radio wave by the CA code (coarse acquisition code) of the target satellite while considering the Doppler effect.
It then receives an almanac and ephemeris data from the target satellite (step 103) Likewise it receives navigation data from other GPS satellites. Recent times have seen systems for complementing the information by receiving the navigation data by way of another wireless network.
Then it calculates the pseudo-distance to each satellite from a radio wave emission clock time and radio wave receiving clock time for each satellite (step 104) and calculates the position of the measuring point by using the navigation data and the pseudo-distance of each satellite (step 105).
The equation used for the calculation in the step 105 is the following, defining the position of the measuring point by the coordinates (x, y, z), and the positions of acquired i-th GPS satellites by the coordinates (xi, yi, zi) for instance:√{square root over ((x−xi)2+(y−yi)2+(z−zi)2)}{square root over ((x−xi)2+(y−yi)2+(z−zi)2)}{square root over ((x−xi)2+(y−yi)2+(z−zi)2)}+Cb=Ri   (1);
where Cb expresses an amount caused by the clock difference between the satellite and the information processing apparatus, and Ri expresses the pseudo-distance between the i-th satellite and the measuring point. The coordinates (xi, yi, zi) can be figured out from the ephemeris data of the respective satellites. Therefore, a solution is obtainable with the number of acquired satellites becoming no less than four because there are four unknowns in the equation, i.e., x, y, z and Cb. If a measuring point is restricted to the surface of the earth, a solution is obtainable with the number of acquired satellite being three.
As described above, the stand alone positioning generally requires the acquisition of four GPS satellites, but it is difficult to acquire four satellites simultaneously in a metropolis such as Tokyo, Japan, with many skyscrapers (i.e., super tall buildings) towering in places.
Let the case of measuring point moving from point A to point B on a road surrounded by a cluster of skyscrapers 201 through 206 as exemplified by FIG. 2 be considered. In this case, the number of acquired satellites is considered to increase at the points A and B since the four directions are visible, while the number of acquired satellites will be limited to the direction the same as the line connecting the points A and B during the time of moving from the point A to point B since the clusters of skyscrapers 202 and 205 are in the way. This prevents acquisition of the necessary number of satellites, resulting in an incapability of positioning.
In order to solve this problem, a car navigation system, et cetera, carries out complementary processing by an integrated use of techniques such as a gyro sensor, gradient sensor, vehicle speed sensor, map matching. Such techniques, however, are not yet practical when considering application to a mobile terminal represented by a mobile phone or a PHS.
Accordingly, a reduced number of GPS satellites required for positioning is desired for locations such as streets in the midst of skyscrapers. The use of the Doppler effect associated with the movement of satellites can be considered as such a method for reducing the number of acquired satellites.
There is a known technique for approximating a current position based on received signals from two satellites through the use of the Doppler effect (e.g., refer to patent document 1 below). The method disclosed thereby, however, is not capable of positioning a measuring point accurately.
Patent document 1: Japan patent application publication No. 2000-235067